The present invention is related to liquid crystalline blends, devices thereof and methods thereof. It finds particular application in conjunction with an organic semiconducting material, a photovoltaic device such as a bulk heterojunction photovoltaic cell, a solar cell, a homeotropically aligned blend thin film, a liquid crystalline blend thin film, a photo-sensitive electric resistor, and an organic light emitting device; and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
In the long term, solar energy is the only source of renewable energy that has the capacity to fill humanity's technological needs. A grand challenge is to convert solar energy into green electric energy in an inexpensive and efficient way. The crystalline silicon photovoltaic cells, though efficient, appear to be too expensive to compete with primary fossil energy.
Organic photovoltaic (OPV) technology would hold a great promise for the cost reduction since the OPV materials are potentially cheap, easy to process, and capable of being deposited on flexible substrates such as plastics and bending where their inorganic competitors e.g. crystalline silicon would crack. Currently widely used OPV materials, e.g. Cu phthalocyanine, suffer from the scattering of electron/exciton between small crystal grain boundaries in which random arrangement of molecules results in poor charge mobility. The attainment of large-area single crystals or desired molecular arrangement of either inorganic (e.g. silicon) or organic molecules is difficult and costly to process although a crystalline phase has superior properties compared to the same material in an amorphous phase. A challenge for OPV, with the possibility of very significant cost reduction, is to make them in desired macroscopic order to improve charge transportation etc. One route to overcoming this problem is to synthesize OPV materials that exhibit liquid crystal (LC) phases since LCs are typically “soft”, i.e., they respond easily to external stimuli and their alignment could be manipulated by external fields and surface effects. LC systems are unique in their partial ordering. In the LC state, these materials are able to self-repair any misorientations and structural defects, which could result in obtaining ordered thin films essential for highly efficient photovoltaic devices.
Organic and polymer semiconducting materials usually have an approximately 10 nm exciton diffusion range. So, only the excitons close to the donor-acceptor interface in a donor and acceptor bilayer photovoltaic cell can contribute to the photocurrent, which dramatically limits photovoltaic performance. Compared with a donor and acceptor bilayer PV cell, a blend made from an electron-donor component and an electron-acceptor component can offer a much larger interface between donor and acceptor as a result of an efficient dissociation of excitons in the supramolecular arrangement. An important improvement on the performance in polymer photovoltaic cells has been demonstrated by mixing an electron donor component (p-type semiconducting material) with an electron acceptor component (n-type material). However, one challenge that remains for OPV technology, with the possibility of very significant cost reduction, is to make them in desired macroscopic order to improve charge transportation.
It is well established that discotic LCs as active components in high efficient photovoltaic (PV) cells are critically dependent on the supramolecular arrangement of the blend made from at least an electron donor component and at least an electron acceptor component. In order to make a discotic LC with more efficient absorption of sunlight, one should consider porphyrin as the building block of the potentially most viable discotic material since it is the basic structure of the best photoreceptor in nature, chlorophyll. Porphyrin and its derivatives have many desirable features such as highly conjugated plane, high stability, intense absorption of sunlight, and the small gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level.
However, to date there is no report on the alignment of discotic LC (such as porphyrin)-fullerene blend or fluorinated LC-fullerene blend, particularly when the ratio between the two is in the range of 1:20 to 20:1. Advantageously, the present invention provides a liquid crystalline blend, a device made using the same and a method of making the same, which meet this need, wherein the LC blend and the device incorporating the LC blend exhibit merits such as favorable molecular arrangement with more interface between electron donating material and electron accepting material, and a viable path for dissociation and electrons and/or holes, among others.